To supplement or even replace conventional fuels derived from decreasing petroleum supplies, fuels formed from renewable sources, particularly biological sources (i.e., so-called “biofuels”), are being sought and developed. Currently, biofuels, such as ethanol, are produced largely from grains, but a large, untapped resource of plant biomass exists in the form of lignocellulosic material. This untapped resource is estimated to encompass more than a billion tons per year (see U.S. Department of Energy (2011) U.S. Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry, Perlack and Stokes, ORNL/TM-2011/224, Oak Ridge National Laboratory, Oak Ridge, Tenn., p. 227—available online at http://www1.eere.energy.gov/biomass/pdfs/billion_ton_update.pdf). Although age-old processes are available for converting the starch content of grain into sugars, which can then be converted to ethanol, the conversion of lignocellulose to biofuel is much more difficult.
Pyrolysis is a thermochemical processing option for producing liquid transportation fuels from biomass. Traditional biomass flash pyrolysis processes have demonstrated a roughly 70% liquid product yield; however, this pyrolysis oil product has limited use without additional upgrading or refining. Current, commercial biomass pyrolysis processes are primarily used to produce commodity chemicals for the food products industry. Fuel uses for raw pyrolysis oils have been demonstrated for electric power production in boilers, diesel engines, and (with limited success) in turbines.
Biomass pyrolysis is the thermal depolymerization of biomass at modest temperatures in the absence of added oxygen to produce a mixture of solid, liquid, and gaseous products depending on the pyrolysis temperature and residence time. Charcoal yields of up to 35% can be achieved for slow pyrolysis at low temperature, high pressure, and long residence time. Flash pyrolysis is used to optimize the liquid products as an oil known as bio-crude or bio-oil. High heating rates and short residence times enable rapid biomass pyrolysis while minimizing vapor cracking to optimize liquid product yields with up to about 70% efficiency on a weight basis.
Bio-oil can be upgraded either at the source prior to full production or after the formation of the liquid product. To date, the two most popular methods in post-production upgrading are adapted from traditional hydrocarbon processing. These processes are bio-oil cracking over solid acid catalysts and hydrotreating in the presence of high pressure hydrogen and a hydrodesulfurization (HDS) catalyst. Although both of these processes have the potential to bring down the oxygen content to a desirable level, it should be noted that both cracking and hydrotreating are accompanied by the loss of hydrogen (as H2O) and carbon (as CO2 or CO) from the bio-oil.
Hydrodeoxygenation (HDO) is carried out at high temperatures (200 to 450° C.) and in the presence of a typical HDO catalysts, most commonly CoMo or NiMo sulfide catalysts. Loss of hydrogen as water during hydrotreating significantly lowers the hydrogen content of bio-oil. In order to offset this, hydrogen typically is externally added during the process at high pressures (e.g., 3 to 12 MPa). As a result, external hydrogen demand can be high—e.g., calculated to be on the order of 41 kg per ton of biomass. Since hydrogen is added to the process at some cost, such a high hydrogen demand makes HDO uneconomical. HDO can be conceptually characterized as follows:C6H8O4+6H2→6CH2+4H2OC6H8O4+4.5H2→6CH1.5+4H2O
Cracking reactions in bio-oils can occur at atmospheric pressure using an acid catalyst. In catalytic cracking, deoxygenation can take place as a result of one or more of dehydration, decarboxylation, and decarbonylation reactions. Decarboxylation specifically leads to the increase in hydrogen-to-carbon (H/C) ratio, thereby increasing the heating value or energy density. Dehydration and decarboxylation reactions can be controlled by modifying the reaction temperature. In general, lower temperatures favor a dehydration reaction, whereas higher temperatures favor a decarboxylation reaction.
Many catalysts have been exploited for the catalytic cracking of pyrolysis oils including zeolites (e.g., H-ZSM-5 and ultrastable Y-zeolite), mesoporous materials like MCM-41 and Al-MCM-41, and heteropolyacids (HPAs). The main disadvantage associated with heteropolyacids is that they are fairly soluble in polar solvents and lose their activity at higher temperatures by losing structural integrity. Major components of bio-oils (phenols, aldehydes, and carboxylic acids) have low reactivity on ZSM-5 and undergo thermal decomposition producing coke.
Zeolite catalysts also deactivate quickly by coke formation from the decomposition of large organic molecules present in the bio-oil. This blocks the pores and decreases the number of available catalytic sites. The large amount of water vapor in bio-oils also leads to dealumination of zeolite materials causing loss of surface area and irreversible deactivation. In comparison, catalytic cracking is regarded as a cheaper route of converting oxygenated feedstocks to lighter fractions. This process, however, leads to higher coke formation (about 8-25 wt %). Unlike the petroleum crude oil upgrading, upgrading of high oxygen content (about 35-50 wt % on a dry basis—i.e., excluding oxygen from any water that may be present) bio-crude into suitable quality biofuels using traditional catalysts will result in significant weight loss of hydrogen and carbon and subsequently decrease the conversion efficiency. During these processes, only a fraction of the carbon present in the raw bio-oil ends up in the upgraded bio-oil. Losses to carbon oxide, and carbon deposition on the catalyst, and system fouling substantially reduce the biomass carbon conversion to final products when upgrading fast pyrolysis bio-oil.
Similar to petroleum crude oil processes, key issues such as coke deposition and catalyst stability still remain for biomass processing or bio-crude upgrading over the conventional catalysts. In some cases, the conventional catalysts may no longer be suitable for bio-crude or biomass processing. For example, due to low sulfur content in the initial biomass feedstock, the conventional sulfided CoMo HDS catalysts used extensively for hydroprocessing in oil refining may not be suitable for bio-crude hydrotreating. The low sulfur environment may cause the reduction of sulfided Co or Ni catalysts to the metal state followed by rapid coke deposition and catalyst deactivation. The necessity to add sulfur donor compounds to the feedstock to maintain the catalytic activity, however, may complicate the process and potentially add sulfur to the fuel product. Cracking over acidic catalysts like zeolites and supported metal oxides (Al2O3), which have the tendency to undergo rapid deactivation due to coking, leads to relatively high yields of light hydrocarbons. Thus, an improved or novel catalyst with better stability for coke formation resistance and higher selectivity towards bio-oil formation will be needed for biomass conversion to bio-oil.
Using dehydration of a fast pyrolysis bio-oil to achieve removal of oxygen (the main product of HDO and cracking over acid catalysts) would require over 80% of the hydrogen in the bio-oil if no external hydrogen were supplied. As a result, a significant amount of hydrogen input is needed to make up for the hydrogen loss as water and thus increase the H/C ratio to a value in the range of 1.9 to 2.4. For example, approximately 20 to 45 kg of hydrogen is required for one ton of biomass to achieve a theoretical yield of 75 to 98 gallons of biofuel per ton of biomass. A number of analyses reveal that upgrading of bio-crude through hydrotreating is not economically attractive because of the high demand of hydrogen. It can also be seen that similar issues will occur to the upgrading of bio-crude through conventional cracking over acid catalysts. Therefore, conventional methodologies such as hydrotreating and cracking do not allow higher efficiencies to be achieved during the conversion of biomass to upgraded bio-oil. In order to achieve high conversion efficiencies, a catalytic biomass pyrolysis process that selectively deoxygenates the biomass with minimal hydrogen and carbon loss can be advantageous. Thus, there remains a need in the art for useful processes for transformation of biomass into high value commodities and/or stable intermediates therefor.
Recent studies have detailed the potential of catalytically upgrading condensed bio-oil into gasoline range hydrocarbons. For example, U.S. Pat. Pub. No. 2009/7578927 to T. Marker et al. describes work with the National Renewable Energy Laboratory (NREL) and the Pacific Northwest National Laboratory (PNNL) for developing a two-stage hydrotreating process to upgrade raw bio-oil into gasoline and diesel. This work focused on separating the pyrolytic lignin fraction of whole bio-oil, blending this fraction with vegetable oils and free fatty acids to form a slurry, and injecting the slurry into a hydrotreating reactor/process with nickel catalysts.
Another process option is catalytic biomass pyrolysis to catalytically modify the composition of the bio-crude intermediate to improve the efficiency of the upgrading step. For example, U.S. Pat. Pub. No. 2010/0105970 to P. O'Conner et al. describes catalytic pyrolysis in a three-riser FCC-type process. The process first consisted of mixing a base catalyst with biomass in a pretreatment step and reacting at a temperature of 200 to 350° C. The second step consisted of acid catalyst cracking and deoxygenation at 350 to 400° C. where the products from the first step were added to a reactor with a solid acid catalyst. The process further made use of a regenerator operating at temperatures up to 800° C. to burn the coke deposits on the catalyst and provide process heat.
U.S. Pat. Pub. No. 2009/0227823 to G. Huber described catalytic pyrolysis using zeolites that are unpromoted or are promoted with metals. The pyrolysis was carried out at a temperature of 500 to 600° C. and a pressure of 1 to 4 atm (approximately 101 to 405 KPa) to produce a highly aromatic product.
Publication WO 2009/018531 to F. Agblevor described the use of catalytic pyrolysis to selectively convert the cellulose and hemicellulose fractions of biomass to light gases and leave behind pyrolytic lignin. The methods used H-ZSM-5 and sulfated zirconia catalysts in a fluidized bed reactor to obtain an overall bio-oil yield of 18-21%.